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School of Nursing - BSN: Evidence Based Nursing

Making Sense of the Evidence and Understanding the Levels

Systematic Reviews - Levels of Evidence: not all evidence is created equal (from the Cochrane Collaboration)

  1. Strength of the evidence:  The quality of the evidence is determined by the methods used to minimise bias within a study design. ...
  2. Relevance of the evidence:  How appropriate is the outcome measure for the healthcare problem, and how useful is it in measuring the benefits? ...
  3. Size of effect:  How high is the likelihood that the effect of the treatment will achieve clinically relevant benefits (or harms?
  4. Lower levels of evidence are provided by:
  • non-randomized studies ... where a control group has run concurrently ...
  • non-randomized studies ... where intervention effects are compared ...
  • single case studies.

      5.  Confidence interval (CI):

 Even studies that are perfectly designed and carried out may show variable results because of the play of chance.  CI  covers the likely range of the true effect. ...

This information has been extracted from the Cochrane Collaboration website.  To see the complete document:  http://consumers.cochrane.org/levels-evidence

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Rating System: General Hierarchy of Evidence

Hierarchy of Evidence for Intervention/Treatment Questions (in descending order)

Level I Evidence from a systematic review of meta-analysis of all relevant RCTs
Level II Evidence obtained from well-designed RCTs
Level III Evidence obtained from well-designed controlled trials without randomization
Level IV Evidence from well-designed case-control and cohort studies
Level V Evidence from systematic reviews of descriptive and qualitative studies
Level VI Evidence from single descriptive or qualitative studies
Level VII Evidence from the opinion of authorities and/or reports of expert communities

Table taken from Evidence-Based Practice in Nursing and Healthcare:  A Guide to Best Practice, 2011, page 12.  Print volume, 4th floor, Call #  WY 100.7 M527e 211

The Centre for Evidence-Based Medicine: Levels of Evidence

Hierarchies of evidence from the CEBM.  The three documents linked here should be used together to provide a better understanding:

Study Designs and Critical Appraisal Skills Programme Checklist from the Critical Appraisal Skills Programme (CASP)

Ethics Terms in CINAHL

Pyramid of Evidence